The invention relates to a machine for machining workpieces, especially crankshafts and camshafts, comprising at least one milling tool with internal cutter and two parallel guide systems.
The machining of crankshafts poses great requirements on the processing machines, on the tools, and cutting materials as well as the processing method. The crankshafts, because of their length and their complicated shape, are very instable. However, different surfaces of the crankshaft must be machined with high precision. The machining forces that occur during milling deform the workpiece, and this causes dimensional inaccuracy.
The machining of crankshafts is carried out, for example, on milling machines with internal cutter. The circular-cylindrical mains located on the workpiece axis are machined and, if needed, also the adjoining web surfaces, as well as the circular-cylindrical pins that are eccentric to the axis of the workpiece. U.S. Pat. No. 4,171,654 discloses a milling machine with internal cutter where a crankshaft is clamped at both ends in a headstock with a stationary chuck, respectively. The circular-cylindrical shape of the pins and mains is achieved by a circular orbital movement of the internal cutter; this is achieved by an axis interpolation of a horizontal axis and a vertical axis of the milling head. The milling unit is not movable in the direction of the workpiece axis. For machining the individual bearing locations, the headstocks are moved along guide paths to the desired position. A simultaneous machining of several bearing locations is not possible in this type of machine because only one milling unit is present. The machine configuration with milling heads that are stationary in the Z direction does not enable the simultaneous machining of two bearing locations because such a machining would require a change of the axial spacing of the milling heads.
U.S. Pat. No. 4,305,689 discloses a milling machine with internal cutter that is provided with headstocks having chucks, wherein the headstocks are movable along a guide system in the direction of the workpiece axis. The ends of the crankshafts are clamped in the chucks. They are rotatable about a numerically controlled chuck axis. The milling head that accommodates the internal cutter is provided between the headstocks and is movable along the same guide path on which the headstocks are guided in the direction of the workpiece axis. The internal cutter is additionally movable in the milling head along a vertically positioned axis. Generating a circular-cylindrical shape of the mains and pins is realized by a rotatory movement of the workpiece (axis C) and simultaneous movement of the internal cutter along its vertical axis by axis interpolation. Because of the minimal mounting space between the headstocks, the base of the guide of the milling unit must be kept short in the guiding direction. Its twisting and tilting stiffness is thus significantly reduced; this has a negative effect on the machining precision, the cutting output, the tool service live, and the surface quality. Also, the salient length of the headstocks as a result of the limited mounting space is very large; this causes an additional reduction in the stiffness. In such machine configurations, mounting of the guide path covers is also difficult. They would have to be provided between the two milling units and the headstocks. Depending on the axial movement position, the spacing between these components in an extreme situation are only a few millimeters so that a guiding path cover cannot be provided as a result of the machine configuration. This situation is even more unfavorable when a steady rest that is to be guided also on a guide path is used also. In particular in the case of mass production of crankshafts, a guide path cover is however extremely important. The absence of this component results in machine disruptions and thus in expensive down-times of the machine. Moreover, when a guide path cover is not provided, the cuttings that fall down cause a heating of the guide paths and, as a result of this, dimensional inaccuracy. Such machine configurations are therefore not suitable for precision machining. When the guide path cover is not provided, roller bearing supported linear guides cannot be used because they are very sensitive with regard to dirt particles or cuttings. When particles reach the guide shoes, this causes within a very short period of time great wear up to complete destruction.
U.S. Pat. No. 5,707,187 discloses an internal cutter milling machine where a crankshaft is clamped with both ends in the headstocks with stationary chucks. In addition, a support of the crankshaft by means of a steady rest is provided. Generating the circular cylindrical shape of the pins and mains is realized by an orbital movement of the milling head. This movement of the milling head is generated by coupling the milling unit that is supported so as to be rotatable about two pivot points to two linear actuators that act in different directions. The required axis interpolation is subject to a complicated movement law. The configuration-based large projection of the internal cutter milling machine as well as the support by means of two pivot points requires a solid, heavy construction for obtaining a satisfactory mass stiffness. For obtaining satisfactory machine dynamics, large linear actuators are required which, in turn, causes a large machine footprint. The two pivot points reduce the stiffness of the milling head so that precision machining is not possible. For the orbital movement of the milling tools, sufficient space is required in the direction toward the guide system so that the height of centers of the headstock is relatively large. In this way, the stiffness of the headstocks is reduced enormously for machines of such a configuration. Moreover, the attachment of the guide path covers is possible in this machine only at great expenditure because of the orbital movement of the milling heads and of the pivot points of the milling head suspension.